1. Field of the Invention
The present invention relates generally to antennas and, more particularly, to horizontally polarized omnidirectional antennas.
2. Description of the Related Art
High gain omnidirectional antennas are more difficult to realize with horizontal polarization than with vertical polarization. Vertical radiating elements stacked along a vertical line provide a natural means of achieving high gain with vertical polarization. Radiating elements often have a length of λ/2 or near λ/2. When these elements are rotated to a horizontal mode, the familiar bidirectional dipole azimuth pattern is seen. Turnstile arrays, such as those described in Brown, G. H, “The Turnstile Antenna” Electronics, April 1936, and Masters, D. W., “The Super-turnstile Antenna” Broadcast News, January 1946, were early answers to the challenge of omnidirectional performance with horizontal polarization. Dipoles also have been wrapped into circular (“halo”) shapes, as described, for example, in Stites, F. H., “A Halo for Six Meters” QST, October 1947, or square (“squalo”) shapes to mitigate the pattern; however, gain is reduced. Perhaps the best implementation of circularly wrapped dipoles is the Big Wheel, where three dipoles form a circular array, as described in Mellen, R. H., and Milner, C. T., “The Big Wheel on Two” QST, September 1961. An excellent printed board implementation has been done by Kent Britain (WA5VJB). Basic performance of the “halo” and “big wheel” structures has been extended by use of folded dipole elements. The folded dipole “halo” has been done by Delbert Fletcher (K5DDD) and the folded dipole “super wheel” is credited to Tom Haddon (K5VH). Slots in cylinders or in rectangular wave guides have offered another approach to horizontal polarization at higher frequencies where λ/2 elements become quite small. The work of Cebik and Cerreto should also be mentioned for a three dipole array that yields a far field radiation pattern nearly identical to that of the “big wheel” as a result of the similarity in current distribution on the three dipoles to that of arrays of three dipoles around a circle.
However, to achieve high gain using such antennas requires stacking an array of horizontally polarized unit structures. The feed complexity associated with ten or twelve stacked structures is not trivial. Thus, a horizontally polarized omnidirectional antenna that provides high gain without complicated feed arrangements is needed.
The use of the same reference symbols in different drawings indicates similar or identical items.